Your dmesg(8) output shows that you are using the default 4.3 kernel. By default, OpenBSD does not provide NTFS support, but does offer a driver which provides NTFS support on an experimental basis. This however will require enabling the option in /usr/src/sys/arch/i386/conf/GENERIC, & recompilation of the kernel. For more information, see Section 14.16 of the FAQ:

Recompiling a kernel is definitely beyond my capabilities, at least for the moment. Thank for letting me know; I'll just live without NTFS support (it's not like there's anything important on that drive I can't get to through Windows).